The lentivirus virion infectivity factor (Vif) is an accessory protein that is required for productive replication of the virus in primary cells and some, but not all, transformed T cell lines. HIV-1 that is genetically deficient in Vif (delta-Vif) fails to replicate in nonpermissive cells as the result of the expression of the cell protein APOBEC3G/CEM15. APOBEC3G is a member of the cytidine deaminase family of RNA editing enzymes. Simpler viruses such as murine leukemia virus do not encode Vif. Nevertheless, cloning and functional analysis showed that mouse APOBEC3G is a potent inhibitor of delta-Vif and wild-type HIV-1. Similarly, African Green monkey APOBEC3G was also active against wild-type and delta-Vif HIV-1 but only inhibited delta-Vif but not SIVagm. These findings suggest a species-specific interaction between Vif and APOBEC3G. The goals of this project are to understand the mechanism by which APOBEC3G reduces the infectivity of delta-Vif HIV-1 and to understand how Vif alleviates this inhibition. Human:AGM chimeric APOBEC3Gs will be constructed and used to determine the domain that determines the interaction. HIV-1 will be adapted to replicate in cells expressing non-homologous APOBEC3G. Physical interaction of APOBEC3G and Vif with each other and with cellular factors will be detected by coimmunoprecipitation. To identify a viral editing substrate for APOBEC3G, encapsidated viral RNA, viral mRNA, viral cDNA and proviral DNA will be sequenced from wild-type and delta-Vif virus grown in the presence or absence of APOBEC3G. In addition, to understand the role physiological role of APOBEC3G, knock-out mice will be constructed. Several immune function assays will be used to determine the effects of the deficiency on the immune system. Insights provided by these studies will have implications regarding targeting the APOBEC3G:Vif interaction for the development of novel HIV therapeutics. Such drugs would interfere with the APOBEC3G:Vif interaction without inhibiting APOBEC3G function.